1. Field of the Invention
The present invention relates to a nickel-based catalyst for the anode a fuel cell and in particular, an alkaline fuel cell. The invention also relates to processes for making the catalyst and an anode comprising the catalyst.
2. Discussion of Background Information
A fuel cell (FC) is one of the oldest electrochemical devices that generate electricity, heat and water by direct electrochemical reaction of a hydrogen-rich fuel with oxygen without any harmful emissions and therefore in an extremely environmentally friendly way. The direct generation of electricity allows FCs to be highly energy efficient. FCs have been deployed as an alternative power generation technique for the future in both mobile and stationary applications, ranging from toys to scale power stations and plants, from vehicles to mobile chargers, and from household power to battlefield power. FCs are generally classified according to the nature of the electrolyte: alkaline fuel cells (AFC), proton-exchange membrane fuel cells (PEMFCs), solid oxide fuel cells (SOFCs), phosphoric acid fuel cells (PACFs) and molten carbonate fuel cells (MCFCs), each with its own characteristic, each type requiring particular materials and fuel. However, they all comprise the same essential components, namely anode support, anode catalyst layer, electrolyte, cathode support, cathode catalyst layer, bipolar plates/interconnects and sometimes gaskets for sealing/preventing leakage of gases between anode and cathode. Each fuel cell type also has its own operational characteristics, offering advantages to particular applications. This makes fuel cells a very versatile technology.
AFCs show promise as environmentally friendly electrochemical power sources for distributed cogeneration for building, and transportation applications. The traditional AFCs operate on compressed hydrogen and oxygen and generally use a solution of potassium hydroxide in water as their electrolyte. AFCs use a liquid KOH electrolyte solution because it is the most conductive of all alkaline hydroxides, and also an effective heat transfer and water management medium. In these cells, hydroxyl ions (OH) migrate from the cathode to the anode. The hydrogen fuel is supplied continuously to the anode compartment and an oxidant (often oxygen from air) is fed continuously to the cathode compartment. At the anode, hydrogen gas reacts with the Off ions to produce water and release electrons. Electrons generated at the anode supply electrical power to an external circuit, then return to the cathode. There the electrons react with oxygen and water to produce more OH− ions that diffuse into the electrolyte. AFCs operate at efficiencies up to 70 percent and create little pollution. Because they produce potable water in addition to electricity, they have been a logical choice for spacecraft. The electrical voltage between the anode and the cathode is in the range 0.5-0.9 V, depending on the load and the electrochemical reactions.
For the low and intermediate temperature FCs, platinum and platinum group metals, either as alone (Pt/C) or in combination of two or more thereof, as well as their corresponding alloys have been preferred as ideal materials suitable for use as electrocatalysts for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) due to the high capacity and performance in any media. However, the cost of Pt and the limited world supply are significant barriers to the widespread use of these types of fuel cells. Accordingly, a large-scale production and application of noble metal catalysts in practical FCs is limited due to the scanty world resources and high cost.
Attempts have been made to replace platinum partially or completely by transition metal compositions as anodes for AFCs that would retain performance in fuel cell electrodes without being too expensive. Apart from lower cost, another advantage of transition metal catalysts is their low susceptibility to poisoning.
Palladium is the most attractive replacement for platinum because these two metals have very similar properties (same group of the Periodic Table, same fcc crystal structure, similar atomic size, etc.). The cost of palladium is lower than that of platinum, so it could be a good substitute for platinum as the catalyst in fuel cells. Palladium is also of interest because it is at least fifty times more abundant on the earth than platinum. For these reasons, palladium has been tested in fuel cells as a platinum co-catalyst and/or as a platinum-free anode catalyst in alkaline media. It has been reported that addition of metals like Co, Ru, Mn and especially Ni significantly promotes the activity and stability of Pd/C electrocatalysts.
A variety of inexpensive non-noble metal materials (e.g., Fe, Co, and Ni) for fuel cell components has already been employed as well. The catalytic activity of nickel as the anode catalyst is about three orders of magnitude lower than that of platinum. That performance deficit can be remedied to some extent with a much higher electrode loading of the less expensive catalyst, by enlarging the active area using finely divided particles and otherwise optimizing the electrode structure. With regard to this, nickel-containing composites such as nickel boride (Ni2B) have been widely used as anode materials, because nickel metal is highly conductive and corrosion resistant to potassium hydroxide and demonstrates unique catalytic activity for the hydrogen oxidation reaction (HOR). The Raney-nickel catalyst, which is among the most active non-noble metals for the HOR, has been the target of interest, especially in AFCs. However, electrodes with Raney-nickel catalysts without support have been reported to suffer from insufficient conductivity. Therefore, in order to enhance the electrical conductivity in the catalyst layer and to increase the catalytic activity, Raney-nickel catalysts were alloyed with carbon material. Raney-nickel hydrogen electrodes containing nickel alone as a transition metal component show relatively large polarizations, a time-dependent behavior and high electrolyte diffusion resistance due to low pore volume and small pore size. These problems have been circumvented by doping the starting Ni—Al alloy with a small percentage of transition metals such as Ti, Cr, Fe and Mo prior to extraction by KOH. Different methods of preparations of the active metal both without and with metal dopants (Ti, Mo, Fe, Cr, Cu) have been proposed and reported in the literature aiming to overcome the mentioned difficulties.
Metal hydride (MH) alloys and intermetallic compounds such as MmNi3.5Co0.7Al0.7Mn0.1 and MlNi3.65Co0.85Al0.3Mn0.3 (Ml: La-rich mischmetal) also have been tested for use as anode catalysts in AFCs. They are characterized by good electrochemical properties, mechanical and chemical stabilities in alkaline electrolyte.
According to scientific articles and patents published in recent years, the nickel-containing catalysts continue to be applied as the most promising materials for preparation of the anode electrodes in the H2—O2 AFCs. In some AFCs the anode catalyst material consists of 65% of nickel and 35% of aluminum with 1% of carbon black and 7% of PTFE as bonding agents (see, e.g., JP 2007-087924 A, the entire disclosure of which is incorporated herein). In other AFCs an electrode is obtained by applying a catalyst carrying Ni, Co, and Fe on carbon fine particles to the surface of nickel foam. PTFE is used as a binder (see, e.g., US 2009/0004521 A1, the entire disclosure of which is incorporated herein). In yet another AFC, the anode catalyst layer comprises nickel and the cathode catalyst layer comprises silver. An also present CO2 inhibitor is in the form of a polymer-bound CO2 adsorbent. In particular, the catalyst layer of the anode electrode promotes CO2 desorption (see, e.g., US 2010/0239921 A1, the entire disclosure of which is incorporated herein).
In view of the foregoing, it would be advantageous to be able to replace the platinum metal used for the production of an anode electrocatalyst with a non-platinum metal such as a transition d-metal without significantly decreasing the electrochemical activity of the catalyst.